Massive earthquakes in the Indian Ocean off the coast of Sumatra are just the beginning. Researchers expect a 30-year cycle of mega-quakes like the one that caused the 2004 tsunami.

How can researchers predict earthquakes? By studying coral reefs in the region. Not only are coral reefs many centuries old, but their shape is a direct response to water levels. After a series of earthquakes, usually the reef winds up higher or lower than it was before - and any part of it that's exposed to air dies.

Scientists studying Sumatran reefs say the coral there have experienced massive die-offs as well as new horizontal growth about every two hundred years. Moreover, these changes happened in fits and starts over phases of about 30 - 100 years. That suggests the area experiences what's called an "earthquake supercycle" for several decades every two centuries.

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Last year's 8.4 quake off the coast of Sumatra is probably the first quake in a new supercycle, since the last big die-off in the coral reefs took place in 1833. Other quake cycles hit in 1374, 1596, 1675, and 1797.

Geophysicist Yehuda Bock co-authored a study published in Nature last week that asserted the recent Sumatra quakes were just the beginning. According to Science News:

The region’s 2007 quake released only one-quarter of the energy that had accumulated along this stretch of subduction zone since 1833. So, Bock notes, quakes in the region in the coming decades may be even larger than expected.

“This is the best area in the world to be able to predict a quake,” Bock says. “It’s clear that there’s going to be an event … We just can’t say for sure when it will happen.”